1. Field of the Invention
The present invention relates to a method for determining a black point of a display and a device for determining the black point.
2. Description of Related Art
Display characteristic of a display device depends on a relationship between a value of a signal inputted to the display device and brightness actually obtained on the display device.
The CRT display has a non-linear display characteristic between the value of a signal inputted to the display and brightness actually obtained on the display. More specifically, the display characteristic is as shown in FIG. 1, in which the value of relative brightness changes non-linearly as the value of the input signal varies. The relative brightness is defined as a value of brightness actually attained on the display relative to a maximum brightness which is normalized to a predetermined value of 255. As shown in the figure, the relative brightness decreases as the input value decreases, and reaches zero (0) when the input value reaches a certain value BP which is called as a "black point".
Assuming that each of the input value X and the relative brightness Yd changes among 256 levels 0 to 255, the display characteristic can be approximated by the following exponential function: EQU Yd=0 when X&lt;BP EQU Yd={(X-BP)/(255-BP)}.sup..gamma. .multidot.255 when X.gtoreq.BP
It is noted that the black point BP changes according to a user's control of brightness of the display. The black point BP also changes in time, and according to respective users' visual sensitivities to light. It is therefore necessary to know the black point BP of the display in order to correctly calibrate the display and in order to attain color matching between colors displayed on the display and colors printed by the printer.
In order to determine the black point BP, it is conceivable that the display be controlled to display a test pattern shown in FIG. 2. The test pattern includes a standard brightness region A sandwiched between a pair of identical comparison brightness regions B. The standard brightness region A is formed by an input value X of zero (X=0). The comparison brightness regions B are formed by an input value X which is variable in a range higher than zero (X&gt;0). The user manipulates the device to change the input value X for the regions B until the comparison brightness regions B become lighter than and distinguishable from the region A. In more concrete terms, when the user perceives some slight difference in brightness between the regions A and B, he or she enters this into the device by, for example, pressing a key. The device sets, as the black point BP, the value X which is presently inputted when the input from the user is received.
According to this conceivable method, however, an unskilled user may not determine the black point accurately within a short period of time. It is therefore desirable to provide an improved method for enabling the unskilled user to easily determine the black point.